1. Field of the Invention
The present invention relates to dot matrix printers, and more particularly to dot matrix line printers of the impact type in which the individual hammers of a reciprocating hammer band are selectively impulsed to print lines of characters in dot matrix fashion.
2. History of the Prior Art
It is known in the art to provide dot matrix line printers in which lines of characters are formed by imprinting selected dots within dot matrices on a print paper or other printable medium to form the characters. Such printers may be of the impact type in which hammers, print wires or similar elements movable relative to the paper selectively impact the paper such as through an ink ribbon to print the desired dots. Such printers may also be of the non-impact type such as where an ink jet is periodically directed onto the paper in controlled fashion. Dot matrix line printers offer numerous advantages including relatively high speeds of operation and versatility in the characters that can be printed and the manner in which they are printed.
An example of a dot matrix line printer of the impact type employing a plurality of hammers in a reciprocating hammer bank is provided by U.S. Pat. No. 3,941,051 of Barrus et al, PRINTER SYSTEM, issued Mar. 2, 1976 and assigned to the assignee of the present application. In the printer described in the Barrus et al patent the hammer bank is bidirectionally reciprocated through a fixed distance relative to a length of print paper which is incrementally advanced prior to each sweep of the hammer bank thereacross. Each hammer in the hammer bank prints a selected number of characters in each line by printing a different dot row of the characters with each sweep of the hammer bank across the paper. Modulation bits for the individual hammers are provided by a character generator which stores a different set of bits for each possible character to be printed. Timing of the application of the modulation bits from the character generator to the hammers is controlled by circuitry which responds to sync pulses generated in response to occurrence of the different dot column positions within a line during each sweep of the hammer bank. As each dot column position is entered the series of bytes stored in a recirculating shift register and representing characters to be printed in a given line is recirculated causing the character generator to output the modulation bits corresponding to a particular dot column for each character, the column being determined by the number of the sweep of the hammer bank across the print paper. Application of the outputed modulation bits to the hammers is controlled by counters and related circuitry which first of all select the dot position of each outputed dot row which corresponds to the dot column position of the hammer bank to the exclusion of the other dot positions and secondly selects the bits from the characters which the hammers are addressing to the exclusion of bits from other characters.
The printer described in Barrus et al prints each of the characters within a dot matrix of fixed size. Thus, in one practical example each character is printed within a space in the dot matrix measuring 7 dots high by 6 dots wide. The last three half dot column positions of the 12 half dot positions defining the 6 dot width of such space are not printed in order to provide space between the character and the immediately following character. Consequently each character is printed within a dot matrix measuring 7 dots high and 5 dots wide with the 5 dot width being defined by 9 half dot positions.
For certain applications and for greater versatility of application of printer systems such as that shown in Barrus et al it would be advantageous to be able to vary the size of the characters by varying the size of the dot matrix, and particularly to be able to vary the character size during actual operation of the printer without changing the speed of the hammer bank or other parameters so as to be able to print a given group of characters in their standard size or optionally in a compressed size. For example, during a given printing operation certain forms may dictate the use of compressed print in some or all portions thereof for reasons such as space limitations. Accordingly, it would be desirable to provide a printer capable of printing one or more groups of characters in a plurality of different sizes as the situation demands. The printer should desirably be capable of choosing between the different sizes quickly and during the occurrence of a printing operation and without the need for changing the basic system parameters.